PROJECT SUMMARY Recombination during meiosis serves multiple biological roles. Recombination diversifies genomes by shuffling combinations of mutations, thereby increasing genetic variation and enhancing the efficiency of natural selection. In many species, recombination also ensures that chromosomes segregate properly during gametogenesis. Although these roles should impose strong selective constraints on recombination, recombination rate varies among individuals. This unexpected result raises a major unanswered question: what processes govern variation in recombination rate in nature? Importantly, we still lack a basic picture of how the heritable component of recombination rate varies within and between species ? information that is required for understanding how any phenotype evolves. Two clues about potential determinants of natural variation come from recent studies targeting recombination mechanisms. First, the two sexes present contrasting recombination landscapes and meiotic constraints, raising the prediction that males and females will display discordant patterns of inter-individual variation. Second, there is new evidence that the number of DNA double-strand breaks and the proportion of breaks repaired as crossovers also show differences among individuals, suggesting that these traits could explain natural variation in recombination rate. The proposed research will provide a much-needed portrait of natural genetic variation in recombination rate across multiple evolutionary scales. The contributions of sex and key meiotic processes to variation in recombination rate among individuals will be evaluated. In Aim 1, we will measure polymorphism and divergence in the genome-wide recombination rate during oogenesis and spermatogenesis by applying immunofluorescence cytology to individual mice. Sex-specific, genetic variation in the total number of crossovers will be quantified on geographically global and local scales using a panel of house mice and their relatives. The prediction that recombination rate experiences distinctive evolutionary pressures in the two sexes will be tested through controlled comparisons between females and males across common genomic backgrounds. In Aim 2, we will use immunofluorescence cytology to profile natural genetic variation in molecular processes that lead to crossovers, including the generation of double-strand breaks, the regulation of recombination intermediates, and the assembly of the synaptonemal complex. By linking these traits to the total number of crossovers in the same set of strains, we will test the hypothesis that the decision between crossover and non-crossover repair is a primary factor in recombination rate evolution. Defects in recombination are a leading cause of fetal loss and a leading genetic cause of developmental disabilities in humans. By examining heritable variation in recombination rate and its potential determinants in natural populations of the house mouse ? a model organism for recombination-related disorders ? this project is relevant to human health.